This invention relates to well logging methods and more particularly to nuclear well logging techniques to determine the presence of and magnitude of undesired water flow in cement voids or channels behind steel or other types of casing in cased boreholes. The methods can also be used to quantitatively measure water flowing within a borehole in the presence of other fluids such as oil and gas.
Water flow behind casing between different geological formations has long been a serious threat in the petroleum industry. The flow of water from a nearby water filled formation into a producing petroleum formation creates a serious commercial problem in that the unwanted water is produced with the petroleum liquids and subsequently must be disposed of in an environmentally safe manner. Similarly, saline water can flow into fresh water aquifers by migrating through voids or channels in the cement sheathing of cased wells thereby rendering such fresh water unfit for human consumption in all instances. It is desirable to detect the presence of unwanted water flow behind casing, and also to determine the magnitude of the flow expressed as volume flow rate. Such migration usually occurs through multiple paths and between multiple zones penetrated by a single well borehole. Quantitative volume flow rate measurements allow remedial actions to be applied first to the most serious water flow problems.
Acoustic cement bond logging techniques have been used for many years to estimate the quality of the cement behind casing. Acoustic bond logs in general give an estimate as to the degree of bonding between the casing and the cement sheath, and even forms estimates of the bonding between the cement sheath and the formation penetrated by the well borehole. This type of information can be useful in predicting potential hydraulic communication between zones; however, such measurements cannot yield quantitative water flow measurements. As an example, a cement bond log might indicate a poor casing/cement bond between two nearby zones. If the differential pressure between the zones were large, the volume flow of fluid could be great. If the differential pressure between the zones were small, the actual flow of fluid might be very small and not worth of remedial repair of the cement. Stated another way, in monitoring a cased well borehole for hydraulic communication between zones, it is better to directly and qualitatively measure any water flow rather than to measure parameters related to the physical condition of the cement sheath which might result in flows. Acoustic bond logging provides only the latter information.
A paper by R. M. McKinley, F. M. Bower and R. C. Rumble "The Structure and Interpretation of Noise From Flow Behind Cemented Casing," paper SPE 3999 presented at the SPE-AIME 47th Annual Fall Meeting, San Antonio, Tex., Oct. 8-11, 1972, describes a noise logging technique for locating certain types of fluid movement. The technique primarily measures acoustical noise generated by turbulence from high energy expansion of fluids. Since noise generated is a direct function of energy dissipated in the expansion process, this technique does not appear feasible for detecting water channeling when pressure differentials between zones are too low to generate detectable noise amplitudes. In addition, the measurements are somewhat qualitative.
Another approach to locate void spaces or channels in the cement sheath has been to inject a radioactive tracer substance such as iodine-131 (I-131) or the like through the perforations into the formation and into any void spaces in the cement annulus surrounding the well casing. The theory of this type of operation is that if the tracer material can be forced backward along the flow path of the undesired fluid, its radioactive properties may then be subsequently detected behind the casing by radiation detectors. This type of well logging has usually proven to be at best marginally successful, particularly in loosely consolidated sand formations which is regrettably where undesired fluid communication is most typically encountered. A permeable producing formation itself can absorb most of the tracer material which is forced through the perforations in the casing and cement sheath. Very little, if any, of the tracer material can be forced back along the path of the undesired flow, particularly if this involves forcing the flow of tracer against either formation fluid pressure or upwardly against the force of gravity. Therefore, such tracer logging techniques have been proven marginally effective and, at best, yield only qualitative indications rather than quantitative volume flow rates.
A paper by P. A. Wichmann, E. C. Hopkinson and A. H. Youmans, "Advances in Production Logging," Transactions SPWLA (1967) reported that water flowing outside a steel casing was detected with a well logging tool containing a source of 14 MeV neutrons and two gamma ray detectors. The 14 MeV neutrons interacted with the oxygen-16 (O-16) in the water inducing the radioactive isotope nitrogen-16 (N-16 ), with a 7.13 second half life, by the O-16(n,p)N-16 reaction. Gamma ray activity from the N-16 was detected at two longitudinally mounted detectors positioned "downstream" from the flow yielding not only an indication of flow but the linear flow velocity as well. Linear flow velocity cannot be converted to the desired volume flow rate without a knowledge of the cross sectional area of the flow channel within the cement sheath. This reference does not address this factor and the essential knowledge needed to get an answer.
U.S. Pat. Nos. 4,032,778, '779, '780, and '781 disclose apparatus and methods for determining the linear flow velocity, radial position, direction, and volume flow rate of water flowing within or behind borehole casing. The method is based upon the oxygen activation method discussed previously. The logging tool contains a source of 14 MeV neutrons, at least two gamma ray spectrometers used to detect induced N-16 activity induced by the O-16(n,p)N-16 reaction, a method of determining linear flow velocity from the response of at least two gamma ray spectrometers, a method of determining the radial distance from the center of the logging tool to the center of the flow using the response of at least one gamma ray spectrometer in at least two gamma ray energy ranges, and finally a method of computing volume flow rate within or behind casing using a gross N-16 measurement, the linear flow velocity measurement, and the radial flow position measurement in a predetermined relationship. In order for the gamma ray detectors to record the induced N-16 activity, the flow of water must first pass the source and then the detectors. Direction of flow is, therefore, determined by either reversing the position of the source and two detectors, or using pairs of detectors mounted above and below the source. Excellent results have been obtained using this method over a wide variety of well conditions and flow ranges. The method does, however, require:
1. An oxygen activation background measurement at some depth within the well at which it is assumed that there is no water flow. This background is generated in the environs of the well bore also exposed to the neutron irradiation which, in addition to water, contain the element oxygen (e.g. the formation rock matrices such as SiO.sub.2, CaCO.sub.3, and the cement annulus itself).
2. At least two gamma ray detectors must be mounted longitudinally within the logging tool on the same side of the neutron source. In order to simultaneously determine the direction of the flow, pairs of detectors must be mounted above and below the neutron source therefore requiring a minimum of at least four gamma ray detectors.
3. The gamma ray detectors must be spectrometers. Stated another way, counting rates in two energy ranges must be measured to determine the gross shape of the measured N-16 gamma ray energy versus gamma ray intensity spectrum which, in turn, is used to compute the radial position of the water flow. This requires a relatively sophisticated spectrometer which adds to the complexity, reliability, and cost of the logging tool.
D. C. McKeon, H. D. Scott, J. R. Olesen, G. L. Patton, and R. J. Mitchell, "Improved Oxygen-Activation Method for Determining Water Flow Behind Casing," SPE Formation Evaluation, September, 1991, present an alternate neutron oxygen activation method for measuring water flow behind casing referred to as the impulse-activation technique. This technique utilizes a logging tool with a source of neutrons and at least one gamma ray detector. Fast neutrons emitted by the source with energies of 10 MeV or greater are used to induce N-16 within the water by the O-16(n,p)N-16 reaction. The flowing water and surrounding well bore environs are irradiated with neutrons for a period of 1 to 15 seconds. Induced activation count rate is then measured as a function of time following termination of irradiation for a period of 20 to 60 seconds. Linear flow velocity is determined from the relative maximum of the count rate versus time measurement following radiation. The method of McKeon, et al. exhibits several advantages over the previously discussed method of U.S. Pat. Nos. 4,032,778, '779, '780, and '781. Specifically, the advantages are:
1. The elimination of the need to make a "background" measurement at some point within the well at which it is assumed that no flow exists.
2. The use of a minimum of one gamma ray detector thereby reducing the complexity of the logging tool.
The primary disadvantages of the method of McKeon, et al. is that no analytical method is presented for converting linear flow velocity to volume flow rate. The reference states that such a conversion can be made knowing the cross sectional area of the flow channel. As stated previously, cross sectional area of a channel within the cement annulus cannot be directly measured. The reference also states that, given an estimate of the distance to the center of the flow channel, volume flow rate can be calculated; however, no quantitative method is disclosed for estimating the distance to the center of the flow channel.
R. M. Ostermeier, "Pulsed Oxygen-Activation Technique for Measuring Water Flow behind Pipe," The Log Analyst, May-June 1991, undertook theoretical studies of water flow measurements based upon the impulse-activation methods of McKeon, et al. The work of Ostermeier corroborated McKeon, et al. Ostermeier noted that the distribution of measured activation count rate versus time following neutron irradiation is a function of the radial distance from the center of the logging tool to the center of the flow channel, further stating that the method should provide all information needed "to fully characterize the channel flow." Osteremier does not, however, mention volume flow rate or means for determining radial distance to the center of the flow or how to determine volume flow rate from the measured parameters.